Galectins and Rhamnose-Binding Lectins in Ascidians
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9 Routes in Innate Immunity Evolution: Galectins and Rhamnose-binding Lectins in Ascidians Loriano Ballarin,1 Matteo Cammarata,2 Nicola Franchi1 and Nicolo` Parrinello2 1Department of Biology, University of Padova, Padova, Italy 2Department of Environmental Biology and Biodiversity, University of Palermo, Palermo, Italy 9.1 ANIMAL LECTINS The term ‘lectin’ is commonly used to encompass a wide variety of carbohydrate-binding proteins, widely distributed in viruses, prokaryotes and eucaryotes (Vasta & Ahmed, 2008). The first animal lectins were isolated by Noguchi in early 1900 from Limu- lus polyphemus and Homarus americanus; many years later, Watkins & Morgan (1952) proposed a sugar-specific binding (l-fucose) property for the eel lectin. Animal lectins are grouped in various molecular families, differing in carbohydrate-recognition domain (CRD) structure and organization (Gabius, 1997; Kilpatrick, 2002; Loris, 2002; Vasta et al., 2004). They are involved in a variety of key biological processes, ranging from development (Kaltner & Stierstorfer, 1998; Kilpatrick, 2002) to immune responses (Ara- son, 1996; Vasta et al., 1994). Protein–carbohydrate interactions are the basis of a mechanism for signaling functions, cell communication and self–non-self recognitions and are critical in the establishment and maintenance of highly specific mutualistic asso- ciations in organism–microbe complexes (Sharon & Lis, 1993). In this respect, mutual benefit (symbiosis or commensalism) depends on the maintenance of a tightly regulated balance, whereas colonization of tissues beneficial to the microbe can lead to the loss of host fitness (pathogenesis), unless host-defense responses are able to eliminate the foreign- ness (Casadevall & Pirofski, 2000). Microheterogeneity, originating from multiple lectin gene copies, allelic variation or post-translational modifications of the gene products, expands the molecular diversity and recognition capabilities. The molecular repertoire may provide a broad non-self-recognition capacity for an efficient innate immune recog- nition system based on recognition of carbohydrate moieties. Galectins, rhamnose-binding lectins, C-type lectins, fucolectins, P-type lectins and L-type lectins are some examples of animal lectins (Kilpatrick, 2002; Lopez´ et al., 2011; Shirai et al., 2009). 9.2 ASCIDIANS Ascidians are invertebrate chordates constituting the most-studied and richest in species class of the subphylum Tunicata or Urochordata, which, together with Cephalochordata Marine Proteins and Peptides: Biological Activities and Applications, First Edition. Edited by Se-Kwon Kim. © 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd. 185 186 Marine Proteins and Peptides and Vertebrata, forms the phylum Chordata. The body of the sessile adult is lined by an epidermis and covered with the tunic. The chordate features—notochord, neural tube, muscular tail and pharynx provided with gill slits—are present in the swimming larva and disappear at metamorphosis, with the exception of the pharynx, which extends into the body (Berril, 1955; Burighel & Cloney, 1997). Recent phylogenetic analysis suggests that ascidian could be the sister group of vertebrates (Delsuc, 2006; Tsagkogeorga et al., 2009). The solitary species Ciona intestinalis and the colonial species Botryllus schlosseri are important model organisms for developmental and evolutionary biology studies, including immunobiology. C. intestinalis is widely distributed in the coastal areas of all temperate seas and grows in dense aggregations on any floating or submerged substrate or other fouling organism (Fig. 9.1a). It is an insufficient hermaphroditic broadcast spawner, with a cylindrical soft body attached on a substrate by the posterior end, while at the anterior side the oral and atrial siphons provide for the water flux through the pharynx. The pha- ryngeal sac occupies a wide body region and is formed by bars and vessels containing hemolymph and hemopoietic nodules. Undifferentiated cells can proliferate and differen- tiate the hemocyte lines (Peddie et al., 1995). The genome has been fully sequenced and many transcript sequences of various developmental stages, from embryo to adult, are available in databases (http://genome.jgi-psf.org/Cioin2/Cioin2.home.html). The compound ascidian Botryllus schlosseri (Fig. 9.1b) is a reliable model organism for a variety of studies, ranging from sexual and asexual reproduction to immunobiology (Manni et al., 2007). The complete genome is not yet available and its transcriptome is less known than that of Ciona; however, the number of expressed sequence tags (ESTs) available in databases is progressively increasing due to the efforts of various research groups. Zooid individuals are grouped in star-shaped systems, are enveloped by a com- mon tunic and share the colonial vascular system, with peripheral and radial vessels connecting zooids, buds and budlets. Colonies reproduce asexually and three blastoge- netic generations are usually present in a colony: adult, filtering zooids; buds on zooids; ◦ and budlets on buds (Manni et al., 2007). A weekly (at 20 C) generation change or take-over (TO) allows the cyclical renewal of the colony. Colonial developmental phases lying more than 1 day from the preceding and following TO are collectively referred to as ‘midcycle’ (MC) (Lauzon et al., 1992; Manni et al., 2007). 9.2.1 Inflammatory Responses of the Solitary Ascidian C. intestinalis Various hemocyte types circulating in the pharynx vessels and scattered in a not-vascularized tunic matrix are involved in C. intestinalis innate immunity. Particulate or soluble materials inoculated into the tunic challenge a local inflammatory-like response due to a massive infiltration of hemocytes, which release several products and encapsulate the affected tissue (Arizza & Parrinello, 2009; Cammarata & Parrinello, 2009; Cammarata et al., 2008; Parrinello, 1981; Parrinello et al., 1984a,1984b). In addition, hemocytes are protagonists of the pharynx inflammatory reaction due to lipopolysaccharide (LPS) inoculation (Parrinello, 1996; Parrinello et al., 2007; Pinto et al., 2003; Shida et al., 2003; Vizzini et al., 2007, Vizzini et al., 2008). Humoral factors named ‘tunicate-cytokines’ can stimulate cell proliferation and modulate hemocyte activities (Beschin et al., 2001; Parrinello et al., 2007). Recent progress in the genome sequencing and cDNA/EST production (Azumi et al., 2003; Terajima et al., 2003a,b) derived from hemocytes (Shida et al., 2003; Terajima et al., 2003) has contributed to the study of immunity-gene expression and function. Genes for Routes in Innate Immunity Evolution 187 oral siphon atrial siphon oviduct spermiduct gonad (a) B b V T Z e A (b) Fig. 9.1 (a) Aggregation of C. intestinalis specimens. Scale bar: 2 cm. (b) Ventral view of a B. schlosseri colony. Zooids (Z), buds on zooids (B) and budlets on buds (b) are embedded in the common tunic (T). The endostyle (e) is clearly visible, and many vessels (V) of the common circulation connecting all the zooids can be seen; along the periphery they end with blind ampullae (A). Scale bar: 1 mm. tumor necrosis factor-like proteins (CiTNFα) (Parrinello et al., 2008), C3-like complement factor and CiC3-1a-like fragment (CiC3 and CiC3a) (Pinto et al., 2003), mannose-binding lectin-like (CiMBL, Bonura et al., 2009) and a component of the CAP protein family (cysteine-rich secretory proteins, antigen 5 and pathogenesis-related 1 proteins) can be promptly expressed following LPS inoculation (CiCAP) (Bonura et al., 2010). 9.2.2 B. schlosseri Immune Responses B. schlosseri immune responses include phagocytosis and cytotoxicity, mediated by circulating immunocytes, and finely regulated by cytokines and lectins released by 188 Marine Proteins and Peptides immunocytes themselves (Ballarin, 2008; Menin & Ballarin, 2008). In addition, an inflammatory reaction, including the selective recruitment, extravasation and degran- ulation of cytotoxic cells, with the consequent release of phenoloxidase, a cytotoxic enzyme, and of its polyphenol substrata, is usually observed during the nonfusion reaction between contacting, genetically incompatible colonies (Ballarin et al., 2008). 9.2.3 Ascidian Lectins A wide literature reports on lectin families in tunicates. Multiple lectins of diverse speci- ficities indicate that a very complex lectin repertoire is involved in innate immunity (Queseberry et al., 2003; Sharon & Lis, 2007). Several C-type (Ca-dependent) lectins, both soluble and integral membrane proteins, have been found in ascidians, including mannose- binding lectins, provided with a collectin-like structure, and selectins (Bonura et al., 2009; Green et al., 2006; Raftos et al., 2001; Vasta et al., 1999). A ficolin homolog functions like mammalian collectins (Sekine et al., 2001); pentraxins are acute-phase proteins that, like MBL and other collectins, have opsonic, mitogenic and complement-activation properties (Vasta et al., 1986a,b). In ascidians, a conserved lectin-dependent pathway of complement may be activated and a C3-like component is cleaved by lectin-associated serine proteases (CiMASPs) (Ji et al., 1997; Nonaka & Azumi, 1999; Marino et al., 2002; Miyazawa et al., 2001; Sekine et al., 2001; Vasta et al., 1999). The CiC3a fragment exerts in vitro chemo- tactic activity toward hemocytes (Pinto et al., 2003). Interestingly, a B. schlosseri lectin type contains both a C-type lectin domain and an immunoglobulin-like domain (Pancer et al., 1997), similar to the fibrinogen-related proteins from